Rolling cutters used for earth boring, tunnel boring or reducing rock formations must be designed to survive in a very hostile environment. When in operation, grit developed during the reduction of rocks and earth material has an extremely fine component and is highly abrasive. Because of the fineness of the grit, the abrasive particles are carried as part of the surrounding atmosphere and seals of the prior art have found it difficult, if not impossible, to prevent the ingress of these abrasive particles into the antifriction bearings of the rolling cutters.
The abrasive particles first attack the rotary seal means used by the prior art which then begins to deteriorate rapidly and allows the abrasive particles further ingress into the antifriction bearings and the bearing grease surrounding the antifriction bearings. Once this situation develops, the antifriction bearings fail quite rapidly destroying the entire rolling cutter before an economically useful life can be realized.
Another part of the hostile environment which the rolling cutters must survive in is the considerable shock developed when crushing hard rock formations. The hard rock formations do not reduce uniformly, thereby causing the rolling cutter to traverse a non-uniform path as it moves over the hard rock formations.
A great deal of shock-loading is, therefore, developed which, in turn, causes relative movement of the structural members of the rolling cutter when in operation. Any seal developed for a rolling cutter must, of course, take into account such relative movement and still seal against the fine abrasive particles in the surrounding atmosphere.
Seals have been developed in the prior art that can and do compensate for relative structural movement upon shock-loading and have had some degree of success in keeping the fine abrasive particles from reaching the antifriction bearings.
One such seal of the prior art is known as the "Stellite metallic seal" and is known under the trade name "Cartriseal Alloyseal" and is used in conjunction with an elastomeric material. This prior art seal will be described further in the detailed description of the drawings but a brief description at this point is necessary.
The Stellite metallic seal may be briefly described as two rings of the same diameter and being rectangular in cross section. The rings are made of a material known as Stellite which is a pure, high chrome-high carbon material capable of producing a very high polished surface such as being lapped to 3 microns finish. The extreme flatness and high polish may be put on a side face of each ring such that the two polished surfaces may be abutted together in a sealing arrangement between rotating and nonrotating members.
The highly polished surfaces, when abutted together on the one side and sealed on the other side with an elastomeric seal between the Stellite ring and its respective member, form an effective seal against relative structural member movement within the rolling cutter.
Seals of the above type have been used effectively in the prior art, the Stellite metallic seal being mentioned by way of example only, but there are still major drawbacks of such seals.
A further part of the hostile environment that the rolling cutter operates in involves itself with the operating temperatures of the environment. Heat is developed in the rolling cutters due to the external friction of the rotatable outer body and, also, the action of the comminution of the hard rock formations. Temperatures of around 200.degree. Fahrenheit are commonly encountered in tunnel boring and operations of the like. The Stellite seal, when subjected to temperatures of 200.degree. Fahrenheit and the fine abrasive dust particles, tends to gall and fail more rapidly than at lower temperatures.
Further, the elastomeric material which is acting in cooperation with the Stellite metallic seal is adversely affected, also, and begins to fail in its sealing action.